Data should be gathered only from the businesses you have been working with, which are (now) offering a range of sanitation products. At the start of the project, it is important that businesses are aware of the need for this data to be shared with you - it can be commercially sensitive and some businesses may not be willing to share the information. You will need to check this first, before deciding to use this particular indicator.
Recommended survey
Provide each business with the same data collection form; assure businesses that their data will be kept confidentially and will not be shared externally except for reporting purposes when it will be aggregated (annonymously) with data from all the businesses involved in the project. Data should be gathered on a monthly basis; ideally you would want to continue to have this data, even after the end of your project timeframe